Change Blender Render Save Default to RGB from RGBA
You need to save your preferences as a startup blend-file.

Create new default blend-file
Change the image format settings in Output Properties &gt; Output
Save the blend-file
State the blend-file as the default startup blend-file Info Editor &gt; File&gt; Defaults &gt; Save Startup File

Make sure you're in the saved blend-file when you state the blend as the startup default!
Note: I'm not sure if this overwrites previously saved blend-file defaults.
Unfortunately I think not. But it should for all future saved blends.